A. Field of the Invention
The present invention relates to a new and improved servo-operated throttling control valve employed in liquid and gas flow control applications.
B. Description of the Prior Art
Fluid flow through fluid systems such as a pipe line is typically controlled by a throttling valve including one or more elastomeric expansion elements. The elastomeric elements are biased against one or more ports by control pressure. Opening of the valve is accomplished by reducing the magnitude of the control pressure a sufficient amount below the inlet pressure to allow the upstream fluid pressure to open or expand the elastomeric element away from the ports. This action allows flow through the valve. If the control pressure is further reduced, the valve is further opened thereby increasing the flow through the valve. Flow through the valve may be reduced or terminated by increasing the control pressure to bias the element over all of the ports. A typical expansion element, servo-operated throttling valve is disclosed in U.S. Pat. No. 3,690,344.
Prior art throttling valves typically include a body having a plurality of components resulting in substantial manufacturing costs, a bulky design, and a substantial number of potential leak paths. In addition, prior art throttling valves having an elastomeric expansion element operate by stretching or expanding the element to open the valve. For example, some prior art valves circumferentially stretch the element as much as thirty percent when the valve is fully open. This causes loss of memory of the elastomeric material. Moreover, since the element is physically stretched or expanded for valve opening, substantial lowering of the control pressure is required to achieve full valve opening.
Typical prior art valves are of the wafer type and may be mounted between a pair of pipe flanges in a pipe line. These prior art valves have a cylindrical body sized to fit within a given flange bolt pattern. When the valve is employed in a pipe line having a different flange rating, a different flange bolt pattern must be accommodated. This requires centering devices such as centering tubes or collars to be installed on the bolt studs in order to allow the prior art valve to be employed.
A further element included in prior art expansion element valves is an upstream grill. In these prior art valves fluid flows through the upstream grill prior to entering the throttling region of the valve as defined by the expansion element. When the prior art valve is fully opened, a fluid pressure loss and a resultant velocity increase occurs across the grill. This high velocity fluid flow directly impinges upon the expansion element causing severe erosion due to high pressure, high flow and entrained sand. This severe erosion of the element results in increased service down time and cost. Moreover, the pressure loss results in the necessity to lower the loading pressure substantially in order to obtain full opening of the valve.
In addition, many of the prior art expansion element valves exhibit a high velocity discharge jet in the downstream piping. To correct this high velocity jet, a substantial length of downstream piping is required for the discharge flow to revert from the kinetic to a higher static pressure.